(1) Field of the Invention:
This invention relates to a method for recording a picture such as a wiring pattern or the like by printing the picture on a picture-forming material coated with a film of a photosensitive material, such as a board suitable for use in the fabrication of a printed circuit, in accordance with input picture signals without developing dimensional distortion.
(2) Description of the Prior Art:
Wiring patterns may be formed on insulated boards, which are suitable for use in the fabrication of printed electronic circuits, in accordance with either contact or direct exposure method.
According to the contact exposure method, a board bearing a resist film coated on the surface thereof is exposed to light with a mask film, in which a negative or positive pattern has been formed, kept in contact with the resist film. The thus-exposed board is then developed to fabricate a resist pattern.
The contact exposure method is however accompanied by such drawbacks that it takes first of all several hours to prepare a mask film of a desired picture or pattern for example by means of a coordinate plotter or the like; and dimensional distortion occurs with a wiring pattern formed on an insulated board as such a mask film undergoes shrinkage or expansion and hence develops distortion by variations in temperature and/or humidity. If such a dimensional error arises, some inconvenience would be encountered when drilling the resulting printed circuit in a subsequent step because the positions of at least some holes would be offset from the thus-printed circuit.
Different from the aforementioned conventional contact exposure method, the direct exposure method forms a picture or pattern in the following manner without using any mask film. Namely, two-dimensional picture data are stored as digital picture signals in a memory or the like. The thus-stored picture data are thereafter read out as picture signals. Then, an insulated board, which is suitable for use in the fabrication of a printed circuit, is scanned by light which has in advance been controlled by the picture signals.
FIG. 1 is a block diagram illustrating one example of conventional direct exposure systems.
In a memory 1, there are stored binary picture signals arranged two-dimensionally. The picture signals are controlled by a central processing unit (hereinafter called "CPU" for the sake of brevity) 2 in such a way that they are converted to time-series scanning signals for recording a picture or pattern, which corresponds to the binary picture signals stored in the memory 1, by the scanning technique. The resulting scanning signals are fed to an acousto-optic light modulator 3.
A light beam output from an exposing light beam source, for example, an argon ion laser tube 4 is ON-OFF modulated by the acousto-optic light modulator 3 and is then guided to a polyhedral reflector 7 by way of an expander 9 and fixed mirrors 5,6. Individual reflecting surfaces of the polyhedral reflector 7 are rotated by a motor 8, whereby reflecting and sweeping the exposing light beam in a direction perpendicular to the sheet of the drawing with a prescribed spread angle (sweep angle).
Then, the exposing light beam travels through a focusing lens 10 arranged very close to the polyhedral reflector 7 and is thereafter reflected by a fixed mirror 11 disposed at a position adjacent to an insulated board 12 suitable for use in the fabrication of a printed circuit. The light beam then sweeps and radiates the unexposed insulated board 12 while forming image points thereon.
The insulated board 12 is fixedly mounted on a stage 13, which is movable at a constant speed in a direction perpendicular to the beam-sweeping direction (i.e., in the direction indicated by an arrow O in the figure) owing to the provision of a motor 14 so as to form a subscanning feed mechanism.
When the polyhedral reflector 7 is rotated and the stage 13 is moved, the insulated board 12 is thus plane-scanned successively, at the image point of the exposing light beam, all over the surface thereof.
The accuracy of movement of the stage 13 in the subscanning direction, which movement is achieved by mechanically driving the stage 13, may be maintained at a required level without substantial difficulties in the direct exposure system so long as the motor 14 and force-transmitting mechanism are designed suitably. However, the accuracy of the beam sweepting speed and width which pertain to the rotation of the polyhedral reflector 7, in other words, the accuracy in the main scanning direction is governed by the design and machining preciseness of the optical system which includes the polyhedral reflector 7, focusing lens 10, etc. It is not easy to minimize errors, which are caused by designing and/or machining aspects, to satisfactory levels. Furthermore, it is extremely difficult from the technical viewpoint to maintain the linearity of the angle of rotation of the polyhedral reflector 7 and that of image points along the sweeping line of the light beam with sufficient accuracy.
FIG. 2 is a block diagram of a conventional exposure system which has purportedly overcome the above-described drawbacks.
In FIG. 2, all elements or parts of structure identified by the same reference numerals as those used in FIG. 1 serve in the same manner as their corresponding elements or parts depicted in FIG. 1. Explanation on such elements is thus omitted.
The system illustrated in FIG. 2 is additionally equipped with a narrow grille-like scale 15, photosensor 16 and auxiliary laser tube 17 compared with the system shown in FIG. 1. Furthermore, the fixed mirrors 5,11 have been replaced by half-mirrors 5',11'.
FIG. 3 illustrates a part of the narrow grille-like scale 15 shown in FIG. 2.
In FIG. 2, the narrow grille-like scale 15 is placed in conjugated relation with the point of exposure of the insulated board 12, i.e., the image point on the recording surface of the insulated board 12 relative to the half-mirror 11'.
Similar to the system depicted in FIG. 1, a laser beam output from the argon laser tube 4 is caused to sweep the unexposed insulated board 12 so that the insulated board 12 is exposed to the laser beam. On the other hand, the auxiliary laser tube 17 is arranged in such a way that each laser beam, which is to be output from the laser tube 17, can follow the same optical axis as laser beams output from the argon laser tube 4. The laser beam output from the auxiliary laser tube 17 has a wavelength that is outside a color sensitivity range in which a photosensitive material coated on the insulated board 12 is exposed. The laser beam output from the auxiliary laser tube 17 sweeps the insulated board 12 and, at the same time, also sweeps the front surface of the narrow grille-like scale 15 in a direction parallel to the length of the scale 15.
The laser beam, which has scanned the narrow grille-like scale 15, is then allowed to pass through the openings (see, FIG. 3) of the scale, thereby converted to a laser beam which is repeatedly turned off at a frequency proportional to the sweeping speed. The latter laser beam then enters the photosensor 16.
The photosensor 16 converts the thus-input laser beam to pulse signals in accordance with the sweeping speed. The resulting pulse signals are then input to the CPU 2, which reads out picture signals from the memory 1 in synchronization with the pulse signals. Therefore, a distortion-free picture or pattern is exposed on the insulated board 12.
In the conventional system depicted in FIG. 2, two types of laser beams having different frequencies are used. Even if a focusing lens minimized in chromatic aberration is used as the focusing lens 10, certain residual chromatic aberration (with respect to magnification) still remains. In addition, further aberration may also occur at the half-mirror 11', depending on the incident angle of each laser beam. Accordingly, there is another drawback that the narrow grille-like scale 15, which is illustrated in FIG. 3, has to be formed into non-linear configurations so as to correct such errors.
Since there is a rather long distance from the polyhedral reflector 7 to the insulated board 12 when conducting the exposure of the insulated board 12 by sweeping the laser beam at the polyhedral reflector 7, a further drawback may be developed unless the optical axes of the two laser beams are coincided completely. Namely, a difference may be developed between the scanning points of both laser beams on the insulated board 12, depending on the extent of the beam sweeping angle.